(CNN) - Republican Sen. Rob Portman of Ohio will throw his weight behind Mitt Romney Thursday, just two days ahead of the South Carolina primary.

"I had not planned on endorsing this early in the primary process," the first-term senator said in a statement. "However, I feel one candidate's policy positions, debate performances, and character make him the best candidate to take on President Obama."

Portman, former White House budget director and U.S. trade representative under former President George W. Bush, said he admired all the candidates but felt "convinced" Romney was the right person for the job.

"I feel our chances in November will be improved by coalescing around one candidate and stopping the political attacks among Republicans," he said

With Ohio a likely battleground state in the general election, the senator's name has been speculated as a possible Republican running-mate contender.

A popular conservative who won his senate seat with 57% of the vote in 2010, Portman is most recently known for his role on the 12-member Congressional "super committee" tasked with hammering out a debt deal late last year.

Romney competes in South Carolina's primary on Saturday. Until recently, polls showed Romney with a wide lead over the GOP field in the state, but new surveys indicate former House Speaker Newt Gingrich is gaining ground on the former Massachusetts governor.